The present invention relates to decoding circuitry and techniques, and more particularly, to aircraft selective calling systems and decoders.
In the prior art, there are known selective calling systems in which the system is used to selectively signal a particular vehicle from another location. The calling system can be used in air-to-air or ground-to-air communication and is most typically established by the transmission of a coded multiple tone audio signal for use of by on-board radio receiver. The remote site from which communication with the vehicle is to be initiated, includes a tone source for selecting and transmitting the specific tone sequence which will advise the vehicle of the desired communication and the receiver aboard the vehicle includes a decoder for indicating when a coded tone sequence has been received.
In known systems, the selective calling equipment may have multiple channels, each installed to provide an automatic monitor of prospective communications. Each channel may include the decoder which is coupled to an associated receiver for response to a specific four-tone coded signal generated by the remote site. When the correct coded four-tone signal is received by the decoder, the decoder will provide an indication so that the presence of a transmitted communication is identified in that particular channel.
In such known systems, close spacings of the tone frequencies in the coded sequence and the requirement of the decoder to operate in the presence of electrical interference, requires the use of highly selective, frequency-stable filters for detecting the tone frequencies. In the past this requirement was met by using electro-mechanical resonators, such as vibrating reeds, tuning forks, or lumped-constant LC filters. Such systems, however, have many disadvantages in that the electro-mechanical embodiments are susceptible to mechanical excitation from host vehicle vibrations while the lumped-constant electrical resonators are physically large and require tuning to the specific frequency tones. For systems operating over wide temperature ranges, the Q stability can also be influenced to cause less than maximum system performance.
In certain aircraft selective calling systems, the same have been standardized and are operated on private corporate channels as well as in a world-wide communications network established by ARINC. When used in conventional aircraft, the decoder and associated circuits generally include the ability to provide sixteen selectable tones per channel, wherein each of the tones are derived by the use of active filters. As will be apparent, in multiple channel equipment, there is a necessity for sixteen filters in each channel which greatly increases the size, weight, and complexity of the selective calling system. Even in one channel, a need for the plurality of individual filters increases the cost and complexity of the single channel system and increases the likelihood of component failure because of the need for so many individual components forming the plurality of filters. As a result, there is still a need for the improvement in selective calling systems, and particularly in the decoding circuitry used to decode the multiple tone signals indicating the initiation of communication with a vehicle.
Accordingly, the present invention has been developed to overcome the shortcomings of the above known and similar techniques and to provide an improved selective calling system and decoder of reduced complexity and improved versatility.